Devices for measuring the speed of watercraft have commonly been used in the past. Typically, such devices are mounted on a boat or other watercraft and include an arm or "pick-up" which extends downward into the water. The output signal from the speedometer pick-up, usually an electrical signal, is converted as needed and provided to a speedometer device, such as an analog speedometer, which can be observed by the operator of the watercraft.
For example, in a series of patents issued to Nakamura et al., U.S. Pat. Nos. 4,914,945, 4,914,946, 4,916,644, 4,932,249, 4,956,997, the outboard motor of a watercraft is provided with a forwardly facing opening which experiences dynamic water pressure when the watercraft moves forward through a body of water. The sensed pressure is transmitted through a conduit to a pressure sensor located towards the front of the watercraft. The pressure sensor produces a signal indicative of the pressure which is converted to a speed for display to an operator. FIG. 7 of the '945 patent shows an alternative speed sensor which is a paddle wheel which is rotated by the water as the boat moves forward to provide an indication of speed.
In Hobbs, U.S. Pat. No. 5,110,310, Davis, U.S. Pat. No. 5,142,473, and Refoy, U.S. Pat. No. 4,205,552, various pitot tubes are disclosed for measuring the speed of a boat with respect to the water through which it travels. Such pitot tubes generally extend downwardly from the boat either at the transom or directly from the hull as shown in Refoy. Such pitot tubes have an open end facing directly in the direction of boat movement. Thus, as the boat picks up speed, more pressure is created in the pitot tube as the water is forced against the open end. A pressure transducer is usually located somewhere in the boat as illustrated by the figure in Refoy, and this transducer provides an output proportional to the pressure. The output may then be converted to an appropriate digital or analog signal which is supplied to a speedometer.
Various problems are inherent in such water pick-up systems. The Nakamura et al. system is an expensive system which requires modification of an outboard motor and long pressure conduits extending between the pick-up and the pressure sensor. In addition to the expense, the long pressure conduits do not work well in certain situations since it takes too much time for the pressure to equalize. Thus, the system is not capable of providing a relatively accurate and instantaneous indication of speed. This type of problem is also manifested in the pitot tube type systems which have a longer pressure conduit than is desired for rapid and accurate indications of relative speed.
Also, long pressure conduits are disadvantageous because ambient air pressure can adversely affect the accuracy of the pressure. Such water pick-up systems often must provide mechanical adjustments to compensate for day to day changes in ambient compensate for day to day changes in ambient pressure.
The prior art water pick-ups also have problems with clogging since they use an unprotected opening pointing in the direction of movement. This opening can become obstructed or completely plugged by sand, dirt, vegetation, and other debris. If the hole becomes obstructed or plugged, the speedometer will provide inaccurate readings to the operator. It is extremely important to avoid this situation particularly in water sport competitions where accurate speed measurement is critical.
A different type of speed indicating device is disclosed in Malcolm et al., U.S. Pat. No. 5,007,286 which uses a housing that extends downward at an incline into the water. The housing includes a dynamic pressure sensing transducer disposed in the end of the housing that extends beneath the water line. This pressure sensing transducer is connected to an electrical connector at the base of the housing via wires passing through a center passage of the housing. In this type of system, the water does not enter a conduit to provide pressure against a transducer, but rather, the water moves through an aperture in the end of the device and acts directly against the pressure transducer. One problem with this type of system is that the pressure transducer required is more expensive since it must work beneath the water line and act directly against the force of the water. Another problem is the housing must be large enough to hold the pressure transducer at the end beneath the water. This can cause a substantial disruption of the wake or surface water quality, such as a "rooster tail", and become a problem, particularly in certain water sports such as competition waterskiing.